FIG. 1 shows a bias current generator 100 that can be used in a current mirror circuit. As illustrated, a power supply (not shown) is coupled to a source lead 105 of a p-channel transistor 110. A gate lead 115 and a drain lead 120 of transistor 110 are coupled together via a lead 125. Transistor 110 functions as a diode in this arrangement. Drain lead 120 is coupled to a resistor 130, which is coupled to a reference voltage supply 140 via a lead 135.
Current generator 100 operates by having a power supply voltage V.sub.DD applied to source lead 105. This causes a current I.sub.110 through transistor 110 and a voltage drop V.sub.110 across transistor 110. Since transistor 110 is in saturation, voltage drop V.sub.110 will be a function of current I.sub.110. The voltage at node 145 (V.sub.145) will be constant due to this voltage drop, and equal to V.sub.DD -V.sub.110.
All of current I.sub.110 is applied to resistor 130 to cause a voltage drop across resistor 130 (V.sub.130) equal to I.sub.110 .times.R.sub.130. Yet I.sub.110 .times.R.sub.130 must equal the constant voltage V.sub.145 (V.sub.DD -V.sub.110) at node 145. Any variation of the voltage V.sub.145 will be applied through lead 125 to gate lead 115 to adjust the "turn-on" level of transistor 110. As a result, current I.sub.110 will change so that, eventually, I.sub.110 .times.R.sub.130 equals the voltage at node 145. Hence, a constant current source is provided.
The operation of current generator 100 discussed above is ideal. In other words, variations in power supply voltage, temperature or the fabrication processes will cause current generator 100 to provide different current amounts. In particular, one disadvantage of current generator 100 is that the voltage from a power supply (e.g., V.sub.DD), the temperature or the process variations can cause as much as a threefold change in the value of current I.sub.110. This can cause inconsistent and possibly erroneous operation of a circuit that utilizes current generator 100.
For example, a device including current generator 100 may be used in an environment where the power supply voltage is susceptible to noise. This noise will alter the current provided by current generator 100. Also, that device may be used in applications where the ambient temperatures can be between minus 55.degree. C. to positive 125.degree. C. These temperature variations can cause a change in the current provided by current generator 100, which can have an adverse affect on device performance.
To illustrate, the operation of current generator 100 will be explained for two ambient temperatures. For temperature 1, a steady-state current I.sub.110 ' will be generated. For a temperature 2 that is greater than temperature 1, the resistance of transistor 110 will increase. As a result, the current I.sub.110 will decrease, causing the voltage V.sub.145 to decrease. The decreased voltage V.sub.145 will be applied to the gate of transistor 110 to turn that transistor on harder. Current I.sub.110 will then increase, but still be less than the steady-state current I.sub.110 '. Thus, a constant current will not be generated over a range of temperature variations.
A band gap circuit-based current source can be used to overcome this disadvantage. One such circuit is disclosed in U.S. Pat. No. 5,629,611 to McIntyre entitled "CURRENT GENERATOR CIRCUIT FOR GENERATING SUBSTANTIALLY CONSTANT CURRENT." The drawback to such current source is that it is a physically large circuit due to its use of many circuit elements. See FIG. 2 in the referenced patent. This is unacceptable since silicon area of integrated circuits is costly.
A need exists for a device that will provide a substantially constant current source or sink despite voltage, temperature and process variations. The present invention meets this need.